1. Field of Invention
This invention relates to a metal container which can be used in a microwave oven. In particular, the invention relates to a metal container which is electrically insulated from the oven and other objects therein. The invention also relates to a method for heating an object in a metal container in a microwave oven, and to a method for storing an object In a metal container, then irradiating the object and the container with microwave radiation without damaging the power tube of the oven.
2. Description of Related Art
Use of microwave energy to heat objects is a well-accepted practice. In particular, the heating of food in a microwave oven, both to cook and to raise the temperature of previously-cooked food for consumption, is widely accepted by consumers for use in the home. Use of a microwave oven in the home no longer is a novelty. Indeed, many products now are packaged specifically to provide ease of use in a microwave oven. Also, microwave ovens of ten are placed In "self-service" vending areas, affording one the opportunity to obtain a hot meal in, for example, a public vending area, in the absence of an attended kitchen. Rather, a food product purchased from, e.g., a vending machine, can be heated in a microwave oven to provide a hot meal.
An object to be heated in a microwave oven often is placed in a container. For example, a container is necessary to retain objects which are or become fluid, such as soups and sauces, and to preclude deposition of residue resulting from placing the object directly on the bottom, or floor, of the oven. Further, objects resting directly on the floor of a microwave oven typically do not heat uniformly because the standing wave pattern within the oven is not uniform in areas adja- cent oven boundaries.
Containers made from various materials are suitable for use in a microwave oven. Paper-based products, such as paper towels, napkins, paper plates, cardboard containers, and wax paper, commonly are utilized in a microwave oven. Certain plastic, ceramic, and glass products also are utilized as materials of construction for such containers. Typically, suitable paper, plastic, ceramic, and glass products neither absorb nor reflect microwave energy. Rather, these products are transparent to the energy. Therefore, energy is not utilized to heat the container. Rather, the energy is absorbed by the object and heats the object in the container.
Containers which absorb microwave radiation may not be suitable for use in a microwave oven. Obviously, it would be undesirable if energy absorbed by the container caused the container to melt or otherwise suffer damage. For example, many plastics absorb microwave energy, and would be deformed when exposed to microwave radiation. Similarly, a container made from microwave-transparent material, upon which is applied material which absorbs microwave energy, likely will be unevenly heated. Under such conditions, materials will experience thermal stress and may crack.
However, absorption of energy by the container may not be deleterious or undesirable, depending upon the object to be achieved thereby. If it is desired to use the high temperature of the container to impart a particular property to the object being heated, for example, to brown the surface of food or to maintain the thus-heated object at an elevated temperature, an increase in the temperature of the container is not deleterious.
Use of metal in a microwave oven is gaining acceptance, but only under limited conditions. Bare metal can reflect the electromagnetic energy toward the power tube (magnetron) and damage it. Also, bare metal can cause arcing between the container and other metal objects in the oven, including the oven walls. Although modern microwave ovens typically are designed to minimize damage by reflected microwaves, both reflection of microwaves and arcing preferably are minimized.
One example of an acceptable use of metal in a microwave oven is the use of a small quantity of metal, often aluminium, as a shield for a portion of the object being heated. For example, metal (aluminium) foil can be wrapped around the wings or the legs of fowl, or the end of a roast, being heated in a microwave oven to prevent absorption of a disproportionate amount of energy so that thus-protected parts are not overheated while the remainder of the object is heated. In such uses, the mass of the metal is small compared to the mass of the object being heated. Further, care must be exercised to ensure that the metal is smoothly shaped to prevent arcing.
It has been suggested that foods in containers transparent to microwaves will heat more quickly when irradiated with microwave energy if a sheet of aluminium foil is placed on the top of the food. The foil is said to direct microwaves to the bottom of the dish which helps the food simmer more quickly and evenly. Even though modern microwave ovens are designed to limit damage from reflected energy, use of metal in such a fashion should be limited so that the potential for causing damage to the power tube is reduced. In accordance with the suggestion, the aluminium foil is to be `normal-weight`, rather than `heavy-duty`, and it must be covered with plastic wrap to minimize sparking.
A region of microwave-reflective material may be arranged on a container made from microwave-transparent material, such as paper, paperboard, cardboard, or glass. Such reflective regions may be utilized to shield the content of the container, in whole or in part, from electromagnetic energy. Such a container is disclosed in U.S. Pat. No. 4,703,149. In contrast, a region formed of ferrite material is heated by absorption of microwave energy until the temperature of the material reaches the Curie temperature, above which the material ceases to absorb microwave energy. Other microwave-absorptive material disposed on a microwave-transparent container also is heated by microwave irradiation. Thus, at least in part, heat is imparted to the object by conduction from the heated microwave-absorptive area. Such heating often is used to provide a "browning" effect to food in the container. A container having a microwave-absorptive surface formed by a very thin layer of aluminium is disclosed in U.S. Pat. No. 4,641,005. As disclosed in the patent, although aluminium and other electrically conductive elemental metals typically do not absorb microwave energy, a very thin layer thereof (about 700 angstroms for aluminium) does absorb microwave energy and becomes heated.
Metal containers which are coated to be suitable for use in a microwave oven are disclosed in U.S. Pat. Nos. 4,558,198 and 4,560,850. The patents disclose four criteria which must be satisfied to make a metal container suitable for use in a microwave oven. Both interior and exterior surfaces must be coated with microwave-absorbing material, which is said to preclude potentially deleterious reflection of electromagnetic energy back to the power tube. In addition to these coatings and covering, the shape of the container is strictly controlled. The side walls must be smoothly curved and free of wrinkles. Further, the height of the side walls is limited to about 33 millimeters. The shallow structure is said to minimize side wall height and reduce microwave reflection, allowing more energy to flow into the food and distribute heat more evenly. Importantly, each corner has a "generous" radius of curvature (about 7/8-inch). The top of the container is provided with a smoothly-curved bead having a diameter of about 1/8-inch. This diameter neither promotes arcing nor cracks or breaks the organic coating. However, a heat-resistant, electrically-insulating, microwave-transparent plastic lid covers the outermost edges at the top of the container to physically separate and electrically insulate these edges from the wall of the oven and from other containers in the oven. The lid also serves to trap steam within the container, thus providing additional heating of the object in the container.
In U.S. Pat. No. 4,558,198, although the coating material is described solely as microwave-diffusing, the claims are directed to a coating which is `heat-resistant, electrically insulating plastic . . . being effective to diffuse . . . microwave energy . . . ` Thus, the patent discloses two types of coatings for the outside of the container; one which only diffuses microwave energy, and one which both diffuses microwave energy and insulates electrically.
The container disclosed in U.S. Pat. No. 4,560,850 is similar in shape to the container disclosed in the '198 patent. In addition to the criteria (`generous radii`) relating generally to the shape of the container, the '850 patent discloses formation of a `steam port` in a raised portion of the bottom of the container. This port both allows steam to escape from the container and directs the flow of thus-escaped steam to the bottom of the container to provide additional heat thereto. All surfaces of this container are coated with material which diffuses microwaves, but electrical insulation capability is not required of this coating.
The tendency of a metal container in a microwave oven to arc is said to be reduced by forming the metal portion of the container in accordance with the disclosure of U.S. Pat. No. 4,851,631. This patent discloses that resonance in the metal part of a container causes deleterious effects, including arcing, localized overheating, scorching of food or non-metallic portions of the container, overheating of food near the edge of the metal, and similar problems. According to the patent disclosure, resonance can be reduced by avoiding use of metal portions having dimensions which exacerbate resonance and by forming the metal portions to reduce resonance including, inter alia, overlapping the ends of metal portions of the container to create capacitance therein.
Use of metal containers in a microwave oven has heretofore been limited. Although many microwave-transparent containers have been developed, such containers are often relatively costly or are otherwise unsuitable for selected uses. For example, paper containers often are not sufficiently sturdy and do not provide appropriate vapor barrier properties for use as containers for food products. Thus, not only is it difficult to ensure continuing integrity of the container, but also moist products are subject to dehydration. Glass and ceramic containers are strong and resistant to fluids, but are subject to breakage. Other microwave-transparent materials, such as plastic, often meet objections based on environmental considerations. Also, it is difficult and relatively expensive to provide a tamper-resistant plastic seal which is retortable, as typically is required by food processors.